Question

What was the purpose of the Monroe Doctrine?

Answer 1

keep European nations from interfering in the Western Hemisphere

Answer 2

For the start hit, i will show you:
this is what google says:
The Monroe Doctrine was a U.S. foreign policy regarding domination of the Americas in 1823. It stated that further efforts by European nations to colonize land or interfere with states in North or South America would be viewed as acts of aggression, requiring U.S. intervention.
now, i would add that it was a document of the time when the  European nations were  to colonize land or/and to  interfere with states in North or South America would be viewed as acts of aggression, requiring U.S. territory intervention.
Hope that this helps you! =)